1. Field of the Invention
The present invention generally relates to a pole change induction motor adapted to change the number of poles thereof when the speed thereof is changed between a high speed and a low speed, and, more particularly, to a pole change induction motor adapted so that an operating range is wide and that an operation thereof is stable when the number of poles thereof is changed.
2. Description of the Related Art
FIGS. 6A and 6B are diagrams illustrating a method of changing the number of poles of a squirrel-cage motor. In the case of a squirrel-cage motor, the number of poles thereof is determined by the number of stator poles thereof. Hitherto, there has been a conventional method of changing the number of poles of the squirrel-cage motor, by which different numbers of poles are obtained by changing the connection on the same winding provided in a stator.
The squirrel-cage motor illustrated in FIG. 6 has four coils consecutively provided in the stator. When the speed of the motor is high, the four coils are excited so that. adjacent coils are different poles. That is, as shown in FIG. 6A, the four coils are excited in such a way as to provide N, S, N and S poles from the left-hand side, as viewed in this figure. Consequently, the motor is now a four-pole motor.
On the other hand, when the speed of the motor is low, the four coils are excited so that all of the coils are the same pole, as illustrated in FIG. 6B. Further, at that time, each S pole is virtually formed between adjacent two N poles. Consequently, the motor is now an eight-pole motor.
Further, for example, Japanese Unexamined Patent Application Publication No. 7-336971 official gazette describes that a first inverter to be connected to a pole unchangeable winding group and a second inverter to be connected to a pole changeable winding group are provided in a squirrel-cage motor configured as described above. Moreover, this official gazette discloses that this motor is configured as an eight-pole motor by causing the first inverter and the second inverter to generate output signals of the same phase, and that a four-electrode motor is obtained by reversing the phase of the output signal only of the first inverter.
In the case of the pole change induction motor of such a configuration, when the speed of the motor is low, high torque is obtained by a small current by increasing the number of poles, whereas when the speed of the motor is high, output power is obtained by reducing the number of poles.
However, in the case of the pole change induction motor of such a configuration, there is a fear that an operation thereof is unstable when the number of pole is frequently changed, because the number of poles is changed by synthesizing a voltage from two kinds of voltages and gradually changing the frequency thereof.
In the case of the conventional pole change induction motor of such a configuration, different numbers of poles are obtained by changing the connection on the same winding. Thus, the number of poles can be changed only when the ratio between the number of poles in the case of the low speed to that of poles in the case of the high speed is 1:2. Further, because the motor has the s am e turn ratio, the voltage utilization factor can be merely doubled. Moreover, as described above, there is a fear that an operation of the motor becomes unstable when the revolution speed thereof frequently changes.
This invention is accomplished to eliminate the aforementioned drawbacks of the conventional pole change induction motor.
Accordingly, an object of the present invention is to provide a pole change induction motor that can obtain an arbitrary ratio between the numbers of poles and that can select an arbitrary number of turns thereby to obtain a very wide operating range and stabilize an operation when the number of poles is changed.
To achieve the foregoing object, according to the present invention, there is provided a pole change induction motor that comprises a rotor having a squirrel-cage winding, a stator having at least a set of a 2n-pole first winding and a 2m-pole second winding wound around the same stator core, a first inverter connected to the first winding, and a second inverter connected to the second winding. Thus, the motor can operate in a wide operating range by a small current.
Further, the numbers xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d may be set so that m=2kn. Incidentally, the coefficient xe2x80x9ckxe2x80x9d is a natural number. This prevents an occurrence of interference due to the combination of the numbers of poles.
Moreover, the number of turns of the first winding may differ from that of turns of the second winding. Thus, the motor can operate in a wide operating range.
Furthermore, the pole change induction motor may further comprise revolution speed detection means for detecting the revolution speed of the rotor, first-inverter control means for controlling the first inverter according to the revolution speed, and second-inverter control means for controlling the second inverter according to the revolution speed. Thus, minute control of individual windings can be conducted. Consequently, the efficiency of the motor can be enhanced.
Additionally, the motor may further comprise torque distribution means for receiving torque commands, and for distributing the torque commands into first torque commands to be outputted to the first inverter, and second torque commands to be outputted to the second inverter. Thus, an easy-to-use motor is realized by distribution of torque generation. Moreover, the efficiency of the motor can be enhanced still more.
Further, the torque distribution means may determine a torque distribution ratio according to the revolution speed represented by data that is outputted by the revolution speed detection means. Therefore, a torque distribution ratio can be determined by an easy method. Consequently, the cost of the motor can be reduced.
Moreover, the torque distribution means may be adapted to distribute the torque commands so that the torque commanded by the second torque command, which is distributed to the second inverter, is higher than the torque commanded by the first torque command, which is distributed to the first inverter, when the revolution speed is equal to or lower than a predetermined value, and that the torque commanded by the first torque command is higher than the torque commanded by the second torque command when the revolution speed is equal to or higher than the predetermined value. Thus, a torque distribution ratio can be determined by an easy method. Consequently, the cost of the motor can be reduced still more.
Furthermore, the torque distribution means may be adapted to first assign the torque commands to the winding having a larger number of poles and then assign a shortage of the torque commands to the winding having a smaller number of poles. Thus, the torque commands can be suitably distributed by performing a simple operation.
Additionally, the first winding and the second winding may be wound and fit into the same slot. This prevents the strength of teeth from being degraded owing to increase in the number of slots. Furthermore, this eliminates a drawback that overlapped coil ends hinder the winding of coils.
Further, when the numbers xe2x80x9cmxe2x80x9d and xe2x80x9cnxe2x80x9d are set so that n greater than m, the 2m-pole second winding may be placed at the opening portion side of the slot. Thus, the skin effect occurring during a high speed operation of the motor can be reduced (that is, the reactance thereof is reduced). Consequently, the efficiency thereof is enhanced. Furthermore, the voltage utilization factor thereof is improved. Thus, a higher speed operation thereof is realized.
Moreover, a section of the 2n-pole first winding in the slot may differ from a section of the 2m-pole second winding therein. Thus, excess materials can be removed. Further, the size of the motor can be reduced.